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AERMOD CHANGES AND UPDATES

4.
What is an Air Quality Model?
An
attempt to predict or simulate the ambient
concentrations of contaminants in an area of interest
An Air Quality Model can be as simple as an algebraic
equation or more complex

12.
Evaluation Opportunity in North Dakota
• Mercer County: Antelope Valley Station and Great Plains
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Synfuels Plant
Electrical generating unit sources dominate SO2
emissions – hourly data available
Five SO2 monitors in area within about 10 km of two
nearby “central” sources
Site‐specific PSD quality meteorological data years
available (10‐m tower)
Major SO2 sources within 50 km were modeled
Five recent years of data were used

13.
Dakota Gasification Co.
• Allowable emissions used for all sources, assumed to be
constantly at peak rates
• Receptors placed at monitor sites only, using actual
terrain (even though slopes are < 2%), except to
characterize the spatial concentration pattern
• Four of the five monitors were at elevations near local
stack base, a fifth monitor was about 100 m higher

19.
Gently Sloping Terrain
• AERMOD has unusual prediction result for very low wind,
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stable conditions and low slope
Problem is, in part, caused by very low mixing height that
leads to very compact plume
Mixing height is below building obstacles, which the
model does not know about
Plume stays perfectly level; terrain should not be
considered in these cases
With terrain, result is an unexpected plume impact “hump”
at point of terrain impact

20.
2. AERMINUTE
• Concentrations not calculated in AERMOD for hours with
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calm or missing meteorological data
AERMINUTE supplements calm hours with hourly values
based on two minute ASOS winds
Non-regulatory component of AERMOD
Purpose of AERMINUTE is not to increase conservatism
but to “reclaim” data that was lost due to METAR reporting
in NWS data
Light wind conditions may be controlling factor in some
cases due to limited dilution

24.
3. EPA formula height
• Good Engineering Practice (GEP) stack height
• The stack height at which a plume released from a stack
is not excessively affected by downwash
• Hgep = Hb +1.5L
• Hb = building height above stack base
• L = lesser of building height and projected building width
• Created a discontinuity in the dispersion profile
http://www.epa.gov/apti/course422/ce1.html

25.
3. EPA formula height
Before April 2011
After April 2011
• Before AERMOD v.
• Since AERMOD v. 11059
11059, building
downwash effects were
ignored if stack height
was greater to the GEP
stack height
• Created a discontinuity in
the dispersion profile
model no longer turns off
downwash above the EPA
formula height
• Downwash effects are
included for stack higher
than the GEP stack height

26.
3. EPA formula height
• EPA: GEP height does not represent height when
downwash effects become negligible but 40% increase in
concentrations due to downwash at GEP height
• AERMOD 11059 “miscellaneous” change
• AERMOD no longer turns off downwash above the EPA formula
height
• Issue for facilities designed with the previous downwash
assumptions that if/when modeled again will experience
increase in concentrations
• Pending clarification memo to clarify this change

27.
Conclusion
• Compliance is a Journey
• Knowing about the challenges related to gently sloping
terrain, ARMINUTE, and the EPA formula height can help
devise a strategy to reach your goal